A distributed power supply delivery network or system is shown in FIG. 1A, generally designated as 10. As shown, network 10 includes supply source 12, bias generator 14 and an analog biased circuit array 16. The supply source provides a voltage VAA (for example), which is shown referenced to ground, AGND. The bias generator, which is coupled to the supply source, includes a current mirror, generally designated as 17, for providing bias control to several current sources, or current mirrors 18a through 18n, the latter disposed in circuit array 16. As will be appreciated, current sources 17 and 18a through 18n may each include several MOSFET transistors (for example) that are configured for biasing on/off to provide the distributed current sources shown in the figure.
A good example of a distributed power delivery network is the current mirrors used in an image sensor, in which each column of a pixel array includes one or more amplifiers that consumes a static amount of current when enabled. Thus, current sources 18a through 18n, respectively, may provide the currents required to read-out the n-columns of a pixel array of an imager sensor. It will be appreciated that the amplifiers (for example) may be connected in series with each of the current sources 18a through 18n. These amplifiers are not shown in FIG. 1A, but are shown, for example, in FIG. 2A as elements 23, referred to herein as array circuitry 25, or array circuits 25.
Distributed power supply delivery networks, however, experience a non-linear IR voltage drop, whereby the IR voltage drop reduces the bias control voltage to each successive current source, as the distance from the power supply source is increased. A good example of this is the IR voltage drop to each current source of the column amplifiers of an image sensor. The IR voltage drop profile, which is caused by the ladder of resistors shown in FIG. 1A, reduces the bias control voltage, Vgs, to each current source 18. It will be appreciated that the ladder of resistors may be resistor devices, or may simply be the sheet resistance of metal routing in a wafer.
This IR voltage drop profile in the power supply (positive or negative supplies) creates difficulties in maintaining uniform bias current levels in the distribution array, as the IR voltage drop induces a change in Vgs between the gate and source of each of the current source MOSFET transistors. This variation may have quite large impacts on the performance of these biased current mirror circuits. As shown in FIG. 1B, the bias control voltage, Vgs, is continuously reduced, as one moves further away from supply source 12 (moving from right to left on the figure). As shown, the voltage VAA is reduced relative to the gate voltage, Vgate, present at each MOSFET transistor of a respective current source, as one moves further away from supply source 12.
One solution to this problem may be to reduce the resistance of the power supply delivery network, thereby reducing the extent of the IR voltage drop. This works for some applications, but is limited to available circuit area for power supply routing. Applications requiring dense circuitry, with relatively high performance requirements, struggle against this tradeoff.
Another solution may be to sample the bias control voltage with sampling capacitors distributed throughout the power distribution array. This only works, however, if the sampling is done before the analog array circuitry is turned on to set-up the non-linear IR voltage drop in the power supply network. This solution effectively maintains a static Vgs voltage on the analog circuitry of the array, as the local power supply network affected by the IR voltage drop (the sampled gate bias voltage) mirrors the movement, thereby maintaining a constant Vgs voltage. This solution forces a trade-off in the cell size of the array (column height in image sensors) for noise, sampling accuracy, and lower leakage. It also introduces a potential source for power supply injection of coherent row-noise, due to the row-wise sampling event.
As will be explained, the present invention, among other benefits, provides a better solution to the aforementioned problem, by reducing the non-linearity in control voltages used to bias the multiple power sources of a distributed power delivery network.